Background: CRLX101 consists of a sugar molecule cyclodextrin linked to a chemotherapy drug called camptothecin. The combined molecule or nanoparticle drug conjugate travels through the blood. Once inside cancer cells, the chemotherapy drug is released from the molecule. Olaparib is a drug that may stop cancer cells from repairing the DNA damage caused by chemotherapy. Researchers want to see how safe it is to give CRLX101 and olaparib together and to see how well the combination treats a specific type of lung cancer called small cell lung cancer (SCLC).